Durable optical element

ABSTRACT

A durable optical film or element includes a polymerized structure having a microstructured surface and a plurality of surface modified colloidal nanoparticles of silica, zirconia, or mixtures thereof. Display devices including the durable microstructured film are also described.

RELATED APPLICATION DATA

This application is a continuation of application Ser. No. 12/730,458,filed Mar. 24, 2010, now allowed, which is a continuation of applicationSer. No. 12/402,525, filed Mar. 12, 2009, issued as U.S. Pat. No.7,713,597, which is a continuation of application Ser. No. 11/927,760,filed Oct. 30, 2007, issued as U.S. Pat. No. 7,524,543; which is acontinuation of Ser. No. 11/278,555, filed Apr. 4, 2006, issued as U.S.Pat. No. 7,309,517; which is a continuation of application Ser. No.10/662,085, filed Sep. 12, 2003, issued as U.S. Pat. No. 7,074,463.

BACKGROUND OF THE INVENTION

The present invention relates generally to durable articles. Moreparticularly, the present invention relates to increasing the durabilityof a microstructured bearing article such as, for example, a brightnessenhancing film, an optical lighting film or a reflective element.

Microstructure bearing articles, such as, brightness enhancing films,optical turning films or reflective elements, are made in a variety offorms. One such form includes a series of alternating tips and grooves.One example of such a form is brightness enhancing film, which has aregular repeating pattern of symmetrical tips and grooves. Otherexamples include patterns in which the tips and grooves are notsymmetrical and in which the size, orientation, or distance between thetips and grooves is not uniform.

One drawback of current brightness enhancing films and optical lightingfilms, and the like, is that the tips of the microstructure aresusceptible to mechanical damage. For example, light scraping with afingernail or a hard, relatively sharp edge can cause the tips of themicrostructure to break or fracture. Conditions sufficient to break thetips of prior art microstructures are experienced during normal handlingof brightness enhancing films, such as, in the manufacturing of liquidcrystal displays for laptop computers.

When microstructure peaks are broken, the reflective and refractiveproperties of the affected peaks are reduced and the transmitted lightscattered to virtually all forward angles. Hence, when the brightnessenhancing film is in a display, and the display is viewed straight on,scratches in the brightness enhancing film are less bright than thesurrounding, undamaged area of the film. However, when the display isviewed at an angle near or greater than the “cutoff” angle, the angle atwhich the image on the display is no longer viewable, the scratches looksubstantially brighter than the surrounding, undamaged area of the film.In both situations, the scratches are very objectionable from a cosmeticstandpoint, and brightness enhancing film with more than a very few,minor scratches is unacceptable for use in a liquid crystal display.

SUMMARY OF THE INVENTION

Generally, the present invention relates to durable articles useful fora variety of applications including, for example, optical elements suchas, for example, microstructured films, as well as the displays andother devices containing the microstructured films.

In one embodiment, a durable optical film includes a polymerized opticalfilm structure having a microstructured surface and a plurality ofsurface modified colloidal nanoparticles of silica, zirconia, ormixtures thereof.

In another embodiment, a durable brightness enhancing film includes apolymerized brightness enhancing structure having a plurality of surfacemodified colloidal nanoparticles.

In a further embodiment, a device includes a lighting device having alight-emitting surface and a brightness enhancing article placedsubstantially parallel to the light-emitting surface. The brightnessenhancing article includes a polymerized structure having a plurality ofsurface modified colloidal nanoparticles.

In another embodiment, a durable optical turning film includes a firstsurface and a second surface. An array of prisms is formed in the firstsurface. The array of prisms has a plurality of first prisms, each ofthe first prisms having a first prism angular orientation with respectto a normal to the first surface and a plurality of second prisms, eachof the second prisms having a second prism angular orientation,different from the first angular orientation, with respect to thenormal. The array of prisms has a plurality of surface modifiedcolloidal nanoparticles.

In another embodiment an illumination device includes a lighting sourcehaving a lightguide having a light-emitting surface and an opticalturning film placed substantially parallel to said lightguide. Theturning film having a first surface and a second surface and an array ofprisms formed on the first surface. The turning film disposed with thefirst surface disposed in relation to the light-emitting surface suchthat light rays exiting the light-emitting surface of the lightguideencounter the array of prisms and are reflected and refracted by thearray of prisms such that the light rays exit the turning film via thesecond surface and substantially along a desired angular direction. Thearray of prisms includes a first plurality of prisms, each of the firstplurality of prisms having a first prism configuration, and a secondplurality of prisms each having a second prism configuration, differentthan the first prism configuration. The first prism configuration andthe second prism configuration being such the light rays exiting thesecond surface correspond to a substantially uniform sampling of thelight rays entering the lightguide. The optical turning film comprisinga plurality of surface modified colloidal nanoparticles.

In another embodiment, retro-reflective film includes a retro-reflectivepolymerized structure having a plurality of surface modified colloidalnanoparticles.

In another embodiment, A durable optical element includes a polymerizedoptical element structure having a microstructured surface and aplurality of surface modified colloidal nanoparticles of silica,zirconia, or mixtures thereof.

The above summary of the present invention is not intended to describeeach disclosed embodiment or every implementation of the presentinvention. The Figures, Detailed Description and Examples which followmore particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a schematic view of an illustrative micro-structured articleof the present invention in a backlit liquid crystal display;

FIG. 2 is a perspective view of an illustrative polymerized structurebearing a micro-structured surface of the present invention;

FIG. 3 is a cross-sectional view of an illustrative micro-structuredarticle in accordance with the present invention which has prismelements of varying height;

FIG. 4 is a cross-sectional view of an illustrative micro-structuredarticle in accordance with the present invention which has prismelements of varying height;

FIG. 5 is a cross-sectional view of an illustrative micro-structuredarticle in accordance with the present invention;

FIG. 6 is a cross-sectional view of an illustrative micro-structuredarticle in which the prism elements are of different heights and havetheir bases in different planes;

FIG. 7 is a cross-sectional view of an illustrative micro-structuredarticle in accordance with the present invention;

FIG. 8 is a cross-sectional view of an illustrative micro-structuredarticle in accordance with the present invention;

FIG. 9 is a cross-sectional view of an illustrative micro-structuredarticle in accordance with the present invention;

FIG. 10 is a schematic view of an illumination device including aturning film in accordance with the present invention;

FIG. 11 is a cross-sectional view of a turning film in accordance withthe present invention; and

FIG. 12 is a cross-sectional view of a turning film in accordance withthe present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION

The durable optical elements and process for making durable opticalelements of the present invention are believed to be applicable to avariety of applications needing durable micro-structured film including,for example, brightness enhancing films, optical turning films as wellas the displays and other devices containing the durablemicrostructures. While the present invention is not so limited, anappreciation of various aspects of the invention will be gained througha discussion of the examples provided below.

Brightness enhancing films generally enhance on-axis luminance (referredherein as “brightness”) of a lighting device. Brightness enhancing filmscan be light transmissible, microstructured films. The microstructuredtopography can be a plurality of prisms on the film surface such thatthe films can be used to redirect light through reflection andrefraction. When used in an optical display such as that found in laptopcomputers, watches, etc., the microstructured optical film can increasebrightness of an optical display by limiting light escaping from thedisplay to within a pair of planes disposed at desired angles from anormal axis running through the optical display. As a result, light thatwould exit the display outside of the allowable range is reflected backinto the display where a portion of it can be “recycled” and returnedback to the microstructured film at an angle that allows it to escapefrom the display. The recycling is useful because it can reduce powerconsumption needed to provide a display with a desired level ofbrightness.

Retro-reflective films generally are capable of returning a significantpercentage of incident light at relatively high entrance anglesregardless of the rotational orientation of the sheeting about an axisperpendicular to its major surface. Cube corner retro-reflective filmcan include a body portion typically having a substantially planar basesurface and a structured surface comprising a plurality of cube cornerelements opposite the base surface. Each cube corner element can includethree mutually substantially perpendicular optical faces that typicallyintersect at a single reference point, or apex. The base of the cubecorner element acts as an aperture through which light is transmittedinto the cube corner element. In use, light incident on the base surfaceof the sheeting is refracted at the base surface of the sheeting,transmitted through the respective bases of the cube corner elementsdisposed on the sheeting, reflected from each of the three perpendicularcube corner optical faces, and redirected toward the light source, asdescribed in U.S. Pat. No. 5,898,523, which is incorporated by referenceherein.

For the following defined terms, these definitions shall be applied,unless a different definition is given in the claims or elsewhere inthis specification.

The term “polymer” will be understood to include polymers, copolymers(e.g., polymers formed using two or more different monomers), oligomersand combinations thereof, as well as polymers, oligomers, or copolymersthat can be formed in a miscible blend by, for example, coextrusion orreaction, including transesterification. Both block and randomcopolymers are included, unless indicated otherwise.

The term “refractive index” is defined herein as the absolute refractiveindex of a material which is understood to be the ratio of the speed ofelectromagnetic radiation in free space to the speed of the radiation inthat material. The refractive index can be measured using known methodsand is generally measured using an Abbe Refractometer in the visiblelight region.

The term “colloidal” is defined herein to mean particles (primaryparticles or associated primary particles) with a diameter less thanabout 100 nm.

The term “associated particles” as used herein refers to a grouping oftwo or more primary particles that are aggregated and/or agglomerated.

The term “aggregation” as used herein is descriptive of a strongassociation between primary particles which may be chemically bound toone another. The breakdown of aggregates into smaller particles isdifficult to achieve.

The term “agglomeration” as used herein is descriptive of a weakassociation of primary particles which may be held together by charge orpolarity and can be broken down into smaller entities.

The term “primary particle size” is defined herein as the size of anon-associated single particle.

The term “sol” is defined herein as a dispersion or suspension ofcolloidal particles in a liquid phase.

The term “surface modified colloidal nanoparticles” refers tonanoparticles, each with a modified surface such that the nanoparticlesprovide a stable dispersion.

The term “stable dispersion” is defined herein as a dispersion in whichthe colloidal nanoparticles do not agglomerate after standing for aperiod of time, such as about 24 hours, under ambient conditions—e.g.room temperature (about 20-22° C.), atmospheric pressure, and no extremeelectromagnetic forces.

The term “gain” is defined herein as a measure of the improvement in theapparent on-axis brightness of a display due to a brightness enhancingfilm, and is a property of the optical material, and also of thegeometry of the brightness enhancing film. Typically, the viewing angledecreases as the gain increases. A high gain is desired for a brightnessenhancing film because improved gain provides an effective increase inthe brightness of the backlight display.

The recitation of numerical ranges by endpoints includes all numberssubsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, and 5).

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural referents unless the contentclearly dictates otherwise. Thus, for example, reference to acomposition containing “a compound” includes a mixture of two or morecompounds. As used in this specification and the appended claims, theterm “or” is generally employed in its sense including “and/or” unlessthe content clearly dictates otherwise.

The polymerizable composition is a substantially solvent-free radiationcurable inorganic filled organic composite. The organic phase of thecomposition consists of a reactive diluent, oligomer, crosslinkingmonomer and optionally includes a photoinitiator. The organic componentcan have a refractive index of at least 1.50 for most productapplications and exhibit significant durability in the cured form. Lowerrefractive index compositions, those less than 1.50, are generallyeasier to achieve based on the vast selection of commercially availablematerials in this refractive index region. Lower refractive index resinshave usefulness in some applications those skilled in the art wouldrecognize. High transmittance in the visible light spectrum is alsodesired. Ideally, the composition minimizes the effect of any inducedscratch while optimizing the desired optical properties and maintaininga Tg (glass transition temperature) significantly high enough to avoidother brightness enhancing product failure modes such as those describedin U.S. Pat. No. 5,626,800.

The polymerizable composition also contains inorganic oxide particleswhose size is chosen to avoid significant visible light scattering. Theinorganic oxide particle selected can impart refractive index or scratchresistance increase or both. It may be desirable to use a mix ofinorganic oxide particle types to optimize an optical or materialproperty and to lower total composition cost. The total composition ofinorganic oxide particles, organic monomers and oligomers preferably hasa refractive index greater than 1.56. Use of inorganic oxide filledpolymers allows one to achieve durability unobtainable with unfilledresins alone. The cured composite composition should meet the necessaryproduct properties of durability, high visible light transmittance,optical clarity, high index of refraction, environmental stability, andphoto stability while possessing the uncured composition requirements oflow viscosity, shelf stability (composition should not change chemicallyover time, particles should not settle or phase separate) and are energycurable in time scales preferably less than five minutes, and thecomposition is substantially solvent free. Compositions with highmulti-functional monomer amounts and reactively functionalized inorganicoxide particles maintain the form of the original master as well as theexisting brightness enhancing films available from 3M, Co.

The present invention describes a durable article that includes apolymerized structure having a plurality of surface modified colloidalnanoparticles. The durable article can be an optical element or opticalproduct constructed of a base layer and an optical layer. The base layerand optical layer can be formed from the same or different polymermaterial. The polymerized structure having a plurality of surfacemodified colloidal nanoparticles has the advantage that it can be formedin a solvent-less system.

Surface modified colloidal nanoparticles are present in the polymerizedstructure in an amount effective to enhance the durability and/orrefractive index of the article or optical element. The surface modifiedcolloidal nanoparticles described herein can have a variety of desirableattributes, including for example; nanoparticle compatibility with resinsystems such that the nanoparticles form stable dispersions within theresin systems, surface modification can provide reactivity of thenanoparticle with the resin system making the composite more durable,properly surface modified nanoparticles added to resin systems provide alow impact on uncured composition viscosity. A combination of surfacemodifications can be used to manipulate the uncured and cured propertiesof the composition. Appropriately surface modified nanoparticles canimprove optical and physical properties of the optical element such as,for example, improve resin mechanical strength, minimize viscositychanges while increasing solid volume loading in the resin system andmaintain optical clarity while increasing solid volume loading in theresin system.

The surface modified colloidal nanoparticles can be oxide particleshaving a particle size or associated particle size of greater than 1 nmand less than 100 nm. Their measurements can be based on transmissionelectron microscopy (TEM). The nanoparticles can include metal oxidessuch as, for example, alumina, tin oxides, antimony oxides, silica,zirconia, titania, mixtures thereof, or mixed oxides thereof. Surfacemodified colloidal nanoparticles can be substantially fully condensed.

Silica nanoparticles can have a particle size from 5 to 75 nm or 10 to30 nm or 20 nm. Silica nanoparticles can be present in the durablearticle or optical element in an amount from 10 to 60 wt %, or 10 to 40wt %. Silicas for use in the materials of the invention are commerciallyavailable from Nalco Chemical Co. (Naperville, Ill.) under the productdesignation NALCO COLLOIDAL SILICAS. For example, silicas include NALCOproducts 1040, 1042, 1050, 1060, 2327 and 2329. Suitable fumed silicasinclude for example, products sold under the tradename, AEROSIL seriesOX-50, -130, -150, and -200 available from DeGussa AG, (Hanau, Germany),and CAB-O-SPERSE 2095, CAB-O-SPERSE A105, CAB-O-SIL M5 available fromCabot Corp. (Tuscola, Ill.).

Zirconia nanoparticles can have a particle size from 5 to 50 nm, or 5 to15 nm, or 10 nm. Zirconia nanoparticles can be present in the durablearticle or optical element in an amount from 10 to 70 wt %, or 30 to 50wt %. Zirconias for use in materials of the invention are commerciallyavailable from Nalco Chemical Co. (Naperville, Ill.) under the productdesignation NALCO OOSSOO8.

Titania, antimony oxides, alumina, tin oxides, and/or mixed metal oxidenanoparticles can have a particle size or associated particle size from5 to 50 nm, or 5 to 15 nm, or 10 nm. Titania, antimony oxides, alumina,tin oxides, and/or mixed metal oxide nanoparticles can be present in thedurable article or optical element in an amount from 10 to 70 wt %, or30 to 50 wt %. Mixed metal oxide for use in materials of the inventionare commercially available from Catalysts & Chemical Industries Corp.,(Kawasaki, Japan) under the product designation Optolake 3.

Surface-treating the nano-sized particles can provide a stabledispersion in the polymeric resin. Preferably, the surface-treatmentstabilizes the nanoparticles so that the particles will be welldispersed in the polymerizable resin and results in a substantiallyhomogeneous composition. Furthermore, the nanoparticles can be modifiedover at least a portion of its surface with a surface treatment agent sothat the stabilized particle can copolymerize or react with thepolymerizable resin during curing.

The nanoparticles of the present invention are preferably treated with asurface treatment agent. In general a surface treatment agent has afirst end that will attach to the particle surface (covalently,ionically or through strong physisorption) and a second end that impartscompatibility of the particle with the resin and/or reacts with resinduring curing. Examples of surface treatment agents include alcohols,amines, carboxylic acids, sulfonic acids, phospohonic acids, silanes andtitanates. The preferred type of treatment agent is determined, in part,by the chemical nature of the metal oxide surface. Silanes are preferredfor silica and other for siliceous fillers. Silanes and carboxylic acidsare preferred for metal oxides such as zirconia. The surfacemodification can be done either subsequent to mixing with the monomersor after mixing. It is preferred in the case of silanes to react thesilanes with the particle or nanoparticle surface before incorporationinto the resin. The required amount of surface modifier is dependantupon several factors such particle size, particle type, modifiermolecular wt, and modifier type. In general it is preferred thatapproximately a monolayer of modifier is attached to the surface of theparticle. The attachment procedure or reaction conditions required alsodepend on the surface modifier used. For silanes it is preferred tosurface treat at elevated temperatures under acidic or basic conditionsfor from 1-24 hr approximately. Surface treatment agents such ascarboxylic acids do not require elevated temperatures or extended time.

Representative embodiments of surface treatment agents suitable for thedurable compositions include compounds such as, for example, isooctyltrimethoxy-silane, N-(3-triethoxysilylpropyl)methoxyethoxyethoxyethylcarbamate (PEG3TES), Silquest A1230,N-(3-triethoxysilylpropyl)methoxyethoxyethoxyethyl carbamate (PEG2TES),3-(methacryloyloxy)propyltrimethoxysilane,3-acryloxypropyltrimethoxysilane,3-(methacryloyloxy)propyltriethoxysilane,3-(methacryloyloxy)propylmethyldimethoxysilane,3-(acryloyloxypropyl)methyldimethoxysilane,3-(methacryloyloxy)propyldimethylethoxysilane,3-(methacryloyloxy)propyldimethylethoxysilane,vinyldimethylethoxysilane, phenyltrimethoxysilane,n-octyltrimethoxysilane, dodecyltrimethoxysilane,octadecyltrimethoxysilane, propyltrimethoxysilane,hexyltrimethoxysilane, vinylmethyldiacetoxysilane,vinylmethyldiethoxysilane, vinyltriacetoxysilane, vinyltriethoxysilane,vinyltriisopropoxysilane, vinyltrimethoxysilane, vinyltriphenoxysilane,vinyltri-t-butoxysilane, vinyltris-isobutoxysilane,vinyltriisopropenoxysilane, vinyltris(2-methoxyethoxy)silane,styrylethyltrimethoxysilane, mercaptopropyltrimethoxysilane,3-glycidoxypropyltrimethoxysilane, acrylic acid, methacrylic acid, oleicacid, stearic acid, dodecanoic acid, 2-[2-(2-methoxyethoxy)ethoxy]aceticacid (MEEAA), beta-carboxyethylacrylate, 2-(2-methoxyethoxy)acetic acid,methoxyphenyl acetic acid, and mixtures thereof.

The surface modification of the particles in the colloidal dispersioncan be accomplished in a variety of ways. The process involves themixture of an inorganic dispersion with surface modifying agents.Optionally, a co-solvent can be added at this point, such as forexample, 1-methoxy-2-propanol, ethanol, isopropanol, ethylene glycol,N,N-dimethylacetamide and 1-methyl-2-pyrrolidinone. The co-solvent canenhance the solubility of the surface modifying agents as well as thesurface modified particles. The mixture comprising the inorganic sol andsurface modifying agents is subsequently reacted at room or an elevatedtemperature, with or without mixing. In a preferred method, the mixturecan be reacted at about 85 degree C. for about 24 hours, resulting inthe surface modified sol. In a preferred method, where metal oxides aresurface modified the surface treatment of the metal oxide can preferablyinvolve the adsorption of acidic molecules to the particle surface. Thesurface modification of the heavy metal oxide preferably takes place atroom temperature.

The surface modification of ZrO2 with silanes can be accomplished underacidic conditions or basic conditions. In one preferred case the silanesare preferably heated under acid conditions for a suitable period oftime. At which time the dispersion is combined with aqueous ammonia (orother base). This method allows removal of the acid counter ion from theZrO2 surface as well as reaction with the silane. In a preferred methodthe particles are precipitated from the dispersion and separated fromthe liquid phase.

The surface modified particles can then be incorporated into the curableresin in various methods. In a preferred aspect, a solvent exchangeprocedure is utilized whereby the resin is added to the surface modifiedsol, followed by removal of the water and co-solvent (if used) viaevaporation, thus leaving the particles dispersed in the polyerizableresin. The evaporation step can be accomplished for example, viadistillation, rotary evaporation or oven drying.

In another aspect, the surface modified particles can be extracted intoa water immiscible solvent followed by solvent exchange, if so desired.

Alternatively, another method for incorporating the surface modifiednanoparticles in the polymerizable resin involves the drying of themodified particles into a powder, followed by the addition of the resinmaterial into which the particles are dispersed. The drying step in thismethod can be accomplished by conventional means suitable for thesystem, such as, for example, oven drying or spray drying.

A combination of surface modifying agents can be useful, wherein atleast one of the agents has a functional group co-polymerizable with ahardenable resin. For example, the polymerizing group can beethylenically unsaturated or a cyclic function subject to ring openingpolymerization. An ethylenically unsaturated polymerizing group can be,for example, an acrylate or methacrylate, or vinyl group. A cyclicfunctional group subject to ring opening polymerization generallycontains a heteroatom such as oxygen, sulfur or nitrogen, and preferablya 3-membered ring containing oxygen such as an epoxide.

The optical layer or micro-structured layer can be formed from thelisting of polymeric material described herein. This layer can be formedfrom high index of refraction materials, including monomers such as highindex of refraction (meth)acrylate monomers, halogenated monomers, andother such high index of refraction monomers as are known in the art.See, for example, U.S. Pat. Nos. 4,568,445; 4,721,377; 4,812,032; and5,424,339, all incorporated by reference herein. The thickness of thisoptical or micro-structured layer can be in the range of about 10 toabout 200 microns.

Suitable polymeric resins to form the optical or micro-structured layerinclude the u.v.-polymerized products of acrylate and/or methacrylatemonomers. A suitable resin is the u.v.-polymerized product of abrominated, alkyl-substituted phenyl acrylate or methacrylate (e.g.,4,6-dibromo-2-sec-butyl phenyl acrylate), a methyl styrene monomer, abrominated epoxy diacrylate, 2-phenoxyethyl acrylate, and ahexa-functional aromatic urethane acrylate oligomer, as described inU.S. Pat. No. 6,355,754, incorporated herein by reference.

While most types of energy polymerizable telechelic monomers andoligomers are useful for the present invention, acrylates are preferredbecause of their high reactivity. Generally, formulations useful in thepresent invention contain reactive diluents in the amount useful toattain viscosities conducive to the method described in below. Thepolymerizable composition should be of flowable viscosity that is lowenough that air bubbles do not become entrapped in the composition andthat the full microstructure geometry is obtained. Reactive diluents aretypically mono- or di-functional monomers such as SR-339, SR-256,SR-379, SR-395, SR-440, SR-506, CD-611, SR-212, SR-230, SR-238, andSR-247 available from Sartomer Co., Exton, Pa. Reactive diluents withrefractive index greater than 1.50, like SR-339, are preferred.Oligomeric materials, particularly those with high refractive index, arealso useful in this invention. The oligomeric material contributes bulkoptical and durable properties to the cured composition. Typical usefuloligomers and oligomeric blends include CN-120, CN-104, CN-115, CN-116,CN-117, CN-118, CN-119, CN-970A60, CN-972, CN-973A80, CN-975 availablefrom Sartomer Co., Exton, Pa. and Ebecryl 1608, 3200, 3201, 3302, 3605,3700, 3701, 608, RDX-51027, 220, 9220, 4827, 4849, 6602, 6700-20Tavailable from Surface Specialties, Smyrna, Ga. Additionally, amulti-functional crosslinker is necessary to achieve a durable, highcrosslink density composite matrix. Examples of multi-functionalmonomers include SR-295, SR-444, SR-351, SR-399, SR-355, and SR-368available from Sartomer Co., Exton, Pa. and PETA-K, PETIA and TMPTA-Navailable from Surface Specialties, Smyrna, Ga.

Multi-functional monomers can be used as crosslinking agents to increasethe glass transition temperature of the polymer that results from thepolymerizing of the polymerizable composition. The glass transitiontemperature can be measured by methods known in the art, such asDifferential Scanning Calorimetry (DSC), modulated DSC, or DynamicMechanical Analysis. The polymeric composition can be crosslinkedsufficiently to provide a glass transition temperature that is greaterthan 45° C.

Monomer compositions useful in this invention can have a melting pointthat is below about 50° C. The monomer composition can be a liquid atroom temperature. Monomer compositions useful in this invention can bepolymerized by conventional free radical polymerization methods.

Examples of initiators include, organic peroxides, azo compounds,quinines, nitro compounds, acyl halides, hydrazones, mercapto compounds,pyrylium compounds, imidazoles, chlorotriazines, benzoin, benzoin alkylethers, di-ketones, phenones, and the like. Commercially availablephotoinitiators include, but not limited to, those availablecommercially from Ciba Geigy under the trade designations DARACUR 1173,DAROCUR 4265, IRGACURE 651, IRGACURE 1800, IRGACURE 369, IRGACURE 1700,and IRGACURE 907, IRGACURE 819. Phosphine oxide derivatives arepreferred, such as LUCIRIN TPO, which is 2,4,6-trimethylbenzoy diphenylphosphine oxide, available from BASF, Charlotte, N.C. A photoinitiatorcan be used at a concentration of about 0.1 to 10 weight percent orabout 0.1 to 5 weight percent.

The polymerizable compositions described herein can also contain one ormore other useful components that, as will be appreciated by those ofskill in the art, can be useful in such a polymerizable composition. Forexample, the polymerizable composition can include one or moresurfactants, pigments, fillers, polymerization inhibitors, antioxidants,anti-static agents, and other possible ingredients. Such components canbe included in amounts known to be effective. Surfactants such asfluorosurfactants can be included in the polymerizable composition toreduce surface tension, improve wetting, allow smoother coating andfewer coating defects.

The polymerizable composition can be formed from a hard resin. The term“hard resin” means that the resulting polymer exhibits an elongation atbreak of less than 50 or 40 or 30 or 20 or 10 or 5 percent whenevaluated according to the ASTM D-882-91 procedure. The hard resinpolymer also can exhibit a tensile modulus of greater than 100 kpsi(6.89×10⁸ pascals) when evaluated according to the ASTM D-882-91procedure.

The optical layer can directly contact the base layer or be opticallycoupled to the base layer, and can be of a size, shape and thicknessallowing the optical layer to direct or concentrate the flow of light.The optical layer can have a structured or micro-structured surface thatcan have any of a number of useful patterns as described below and shownin the FIGURES and EXAMPLES. The micro-structured surface can be aplurality of parallel longitudinal ridges extending along a length orwidth of the film. These ridges can be formed from a plurality of prismapexes. These apexes can be sharp, rounded or flattened or truncated.These include regular or irregular prismatic patterns can be an annularprismatic pattern, a cube-corner pattern or any other lenticularmicrostructure. A useful microstructure is a regular prismatic patternthat can act as a totally internal reflecting film for use as abrightness enhancing film. Another useful microstructure is acorner-cube prismatic pattern that can act as a retro-reflecting film orelement for use as reflecting film. Another useful microstructure is aprismatic pattern that can act as an optical element for use in anoptical display. Another useful microstructure is a prismatic patternthat can act as an optical turning film or element for use in an opticaldisplay.

The base layer can be of a nature and composition suitable for use in anoptical product, i.e. a product designed to control the flow of light.Almost any material can be used as a base material as long as thematerial is sufficiently optically clear and is structurally strongenough to be assembled into or used within a particular optical product.Preferably, a base material is chosen that has sufficient resistance totemperature and aging that performance of the optical product is notcompromised over time.

The particular chemical composition and thickness of the base materialfor any optical product can depend on the requirements of the particularoptical product that is being constructed. That is, balancing the needsfor strength, clarity, temperature resistance, surface energy, adherenceto the optical layer, among others.

Useful base materials include, for example, styrene-acrylonitrile,cellulose acetate butyrate, cellulose acetate propionate, cellulosetriacetate, polyether sulfone, polymethyl methacrylate, polyurethane,polyester, polycarbonate, polyvinyl chloride, polystyrene, polyethylenenaphthalate, copolymers or blends based on naphthalene dicarboxylicacids, polycyclo-olefins, polyimides, and glass. Optionally, the basematerial can contain mixtures or combinations of these materials. In anembodiment, the base may be multi-layered or may contain a dispersedphase suspended or dispersed in a continuous phase.

For some optical products such as microstructure-bearing products suchas, for example, brightness enhancing films, examples of preferred basematerials include polyethylene terephthalate (PET) and polycarbonate.Examples of useful PET films include photograde polyethyleneterephthalate and MELINEX™ PET available from DuPont Films ofWilmington, Del.

Some base materials can be optically active, and can act as polarizingmaterials. A number of bases, also referred to herein as films orsubstrates, are known in the optical product art to be useful aspolarizing materials. Polarization of light through a film can beaccomplished, for example, by the inclusion of dichroic polarizers in afilm material that selectively absorbs passing light. Light polarizationcan also be achieved by including inorganic materials such as alignedmica chips or by a discontinuous phase dispersed within a continuousfilm, such as droplets of light modulating liquid crystals dispersedwithin a continuous film. As an alternative, a film can be prepared frommicrotine layers of different materials. The polarizing materials withinthe film can be aligned into a polarizing orientation, for example, byemploying methods such as stretching the film, applying electric ormagnetic fields, and coating techniques.

Examples of polarizing films include those described in U.S. Pat. Nos.5,825,543 and 5,783,120, each of which are incorporated herein byreference. The use of these polarizer films in combination with abrightness enhancing film has been described in U.S. Pat. No. 6,111,696,incorporated by reference herein.

A second example of a polarizing film that can be used as a base arethose films described in U.S. Pat. No. 5,882,774, also incorporatedherein by reference. One example of such films that are availablecommercially are the multilayer films sold under the trade designationDBEF (Dual Brightness Enhancement Film) from 3M. The use of suchmultilayer polarizing optical film in a brightness enhancing film hasbeen described in U.S. Pat. No. 5,828,488, incorporated herein byreference.

This list of base materials is not exclusive, and as will be appreciatedby those of skill in the art, other polarizing and non-polarizing filmscan also be useful as the base for the optical products of theinvention. These base materials can be combined with any number of otherfilms including, for example, polarizing films to form multilayerstructures. A short list of additional base materials can include thosefilms described in U.S. Pat. Nos. 5,612,820 and 5,486,949, among others.The thickness of a particular base can also depend on theabove-described requirements of the optical product.

Durable microstructure-bearing articles can be constructed in a varietyof forms, including those having a series of alternating tips andgrooves sufficient to produce a totally internal reflecting film. Anexample of such a film is a brightness enhancing film having a regularrepeating pattern of symmetrical tips and grooves, while other exampleshave patterns in which the tips and grooves are not symmetrical.Examples of microstructure bearing articles useful as brightnessenhancing films are described by U.S. Pat. Nos. 5,175,030 and 5,183,597,which are both incorporated herein by reference.

According to these patents, a microstructure-bearing article can beprepared by a method including the steps of (a) preparing apolymerizable composition; (b) depositing the polymerizable compositiononto a master negative microstructured molding surface in an amountbarely sufficient to fill the cavities of the master; (c) filling thecavities by moving a bead of the polymerizable composition between apreformed base and the master, at least one of which is flexible; and(d) curing the composition. The master can be metallic, such as nickel,nickel-plated copper or brass, or can be a thermoplastic material thatis stable under polymerization conditions and that preferably has asurface energy that permits clean removal of the polymerized materialfrom the master. The particular method used to create the microstructuretopography described herein can be similar to the molding processdescribed in U.S. Pat. No. 5,691,846 which is incorporated by referenceherein. The micro-structure article according to the invention can beformed from a continuous process at any desired length such as, forexample, 5, 10, 100, 1000 meters or more.

The durable article can be used in applications needing durablemicro-structured film including, for example, brightness enhancingfilms. The structure of these durable brightness enhancing films caninclude a wide variety of micro-structured films such as, for example,U.S. Pat. No. 5,771,328, U.S. Pat. No. 5,917,664, U.S. Pat. No.5,919,551, U.S. Pat. No. 6,280,063, and U.S. Pat. No. 6,356,391, allincorporated by reference herein.

A backlit liquid crystal display generally indicated at 10 in FIG. 1includes a brightness enhancing film 11 of the present invention thatcan be positioned between a diffuser 12 and a liquid crystal displaypanel 14. The backlit liquid crystal display can also includes a lightsource 16 such as a fluorescent lamp, a light guide 18 for transportinglight for reflection toward the liquid crystal display panel 14, and awhite reflector 20 for reflecting light also toward the liquid crystaldisplay panel. The brightness enhancing film 11 collimates light emittedfrom the light guide 18 thereby increasing the brightness of the liquidcrystal display panel 14. The increased brightness enables a sharperimage to be produced by the liquid crystal display panel and allows thepower of the light source 16 to be reduced to produce a selectedbrightness. The brightness enhancing film 11 in the backlit liquidcrystal display is useful in equipment such as computer displays (laptopdisplays and computer monitors), televisions, video recorders, mobilecommunication devices, handheld devices (i.e. cellphone, PDA),automobile and avionic instrument displays, and the like, represented byreference character 21.

The brightness enhancing film 11 includes an array of prisms typified byprisms 22, 24, 26, and 28, as illustrated in FIG. 2. Each prism, forexample, such as prism 22, has a first facet 30 and a second facet 32.The prisms 22, 24, 26, and 28 can be formed on a body portion 34 thathas a first surface 36 on which the prisms are formed and a secondsurface 38 that is substantially flat or planar and opposite the firstsurface.

A linear array of regular right prisms can provide both opticalperformance and ease of manufacture. By right prisms, it is meant thatthe apex angle θ is approximately 90°, but can also range fromapproximately 70° to 120° or from approximately 80° to 100°. The prismfacets need not be identical, and the prisms may be tilted with respectto each other. Furthermore, the relationship between the thickness 40 ofthe film and the height 42 of the prisms is not critical, but it isdesirable to use thinner films with well defined prism facets. The anglethat the facets can form with the surface 38 if the facets were to beprojected can be 45°. However, this angle would vary depending on thepitch of the facet or the angle θ of the apex.

FIGS. 3-9 illustrate representative embodiments of a construction for anoptical element in accordance with principles of the present invention.It should be noted that these drawings are not to scale and that, inparticular, the size of the structured surface is greatly exaggeratedfor illustrative purposes. The construction of the optical element caninclude combinations or two or more of the described embodiments below.

Referring to FIG. 3, there is illustrated a representative cross-sectionof a portion of one embodiment of an optical element or light directingfilm in accordance with the present invention. The film 130 includes afirst surface 132 and an opposing structured surface 134 which includesa plurality of substantially linearly extending prism elements 136. Eachprism element 136 has a first side surface 138 and a second side surface138′, the top edges of which intersect to define the peak, or apex 142of the prism element 136. The bottom edges of side surfaces 138, 138′ ofadjacent prism elements 136 intersect to form a linearly extendinggroove 144 between prism elements. In the embodiment illustrated in FIG.3, the dihedral angle defined by the prism apex 142 measuresapproximately 90 degrees, however it will be appreciated that the exactmeasure of the dihedral angle in this and other embodiments may bevaried in accordance with desired optical parameters.

The structured surface 134 of film 130 may be described as having aplurality of alternating zones of prism elements having peaks which arespaced at different distances from a common reference plane. The commonreference plane may be arbitrarily selected. One convenient example of acommon reference plane is the plane which contains first surface 132;another is the plane defined by the bottom of the lower most grooves ofthe structured surface, indicated by dashed line 139. In the embodimentillustrated in FIG. 3, the shorter prism elements measure approximately50 microns in width and approximately 25 microns in height, measuredfrom dashed line 139, while the taller prism elements measureapproximately 50 microns in width and approximately 26 microns inheight. The width of the zone which includes the taller prism elementscan measure between about 1 micron and 300 microns. The width of thezone which includes the shorter prism elements is not critical and canmeasures between 200 microns and 4000 microns. In any given embodimentthe zone of shorter prism elements can be at least as wide as the zoneof taller prism elements. It will be appreciated by one of ordinaryskill in the art that the article depicted in FIG. 3 is merely exemplaryand is not intended to limit the scope of the present invention. Forexample, the height or width of the prism elements may be changed withinpracticable limits—it is practicable to machine precise prisms in rangesextending from about 1 micron to about 200 microns. Additionally, thedihedral angles may be changed or the prism axis may be tilted toachieve a desired optical effect.

The width of the first zone can be less than about 200 to 300 microns.Under normal viewing conditions, the human eye has difficulty resolvingsmall variations in the intensity of light which occur in regions lessthan about 200 to 300 microns in width. Thus, when the width of thefirst zone is reduced to less than about 200 to 300 microns, any opticalcoupling which may occur in this zone is not detectable to the human eyeunder normal viewing conditions.

A variable height structured surface may also be implemented by varyingthe height of one or more prism elements along its linear extent tocreate alternating zones which include portions of prism elements havingpeaks disposed at varying heights above a common reference plane.

FIG. 4 illustrates another embodiment of the optical element similar toFIG. 3 except that the film 150 includes a structured surface 152 whichhas a zone of relatively shorter prism elements 154 separated by a zoneincluding a single taller prism element 156. Much like the embodimentdepicted in FIG. 3, the taller prism element limits the physicalproximity of a second sheet of film to structured surface 152, therebyreducing the likelihood of a visible wet-out condition. It has beendetermined that the human eye is sensitive to changes in facet heightsin light directing films and that relatively wide zones of taller prismelements will appear as visible lines on the surface of a film. Whilethis does not materially affect the optical performance of the film, thelines may be undesirable in certain commercial circumstances. Reducingthe width of a zone of taller prism elements correspondingly reduces theability of a human eye to detect the lines in the film caused by thetaller prism elements.

FIG. 5 is a representative example of another embodiment of an opticalelement in which the prism elements are approximately the same size butare arranged in a repeating stair step or ramp pattern. The film 160depicted in FIG. 5 includes a first surface 162 and an opposingstructured surface 164 including a plurality of substantially linearprism elements 166. Each prism element has opposing lateral faces 168,168′ which intersect at their upper edge to define the prism peaks 170.The dihedral angle defined by opposing lateral faces 168, 168′ measuresapproximately 90 degrees. In this embodiment the highest prisms may beconsidered a first zone and adjacent prisms may be considered a secondzone. Again, the first zone can measure less than about 200 to 300microns.

FIG. 6 illustrates a further embodiment of an optical element inaccordance with the present invention. The film 180 disclosed in FIG. 6includes a first surface 182 and an opposing structured surface 184.This film may be characterized in that the second zone which includesrelatively shorter prism elements contains prism elements of varyingheight. The structured surface depicted in FIG. 6 has the additionaladvantage of substantially reducing the visibility to the human eye oflines on the surface of the film caused by the variations in the heightof the prism elements.

FIG. 7 shows another embodiment of an optical element according to theinvention for providing a soft cutoff FIG. 7 shows a brightnessenhancing film, designated generally as 240, according to the invention.Brightness enhancing film 240 includes a substrate 242 and a structuredsurface material 244. Substrate 242 is can generally be a polyestermaterial and structured surface material 244 can be an ultraviolet-curedacrylic or other polymeric material discussed herein. The exteriorsurface of substrate 242 is preferably flat, but could have structuresas well. Furthermore, other alternative substrates could be used.

Structured surface material 244 has a plurality of prisms such as prisms246, 248, and 250, formed thereon. Prisms 246, 248, and 250 have peaks252, 254, and 256, respectively. All of peaks 252, 254, and 256 havepeak or prism angles of preferably 90 degrees, although included anglesin the range 60 degrees to 120 degrees. Between prisms 246 and 248 is avalley 258. Between prisms 248 and 250 is a valley 260. Valley 258 maybe considered to have the valley associated with prism 246 and has avalley angle of 70 degrees and valley 260 may be considered the valleyassociated with prism 248 and has a valley angle of 110 degrees,although other values could be used. Effectively, brightness enhancingfilm 240 increases the apparent on axis brightness of a backlight byreflecting and recycling some of the light and refracting the remainderlike prior art brightness enhancing film, but with the prisms canted inalternating directions. The effect of canting the prisms is to increasethe size of the output light cone.

FIG. 8 shows another embodiment of an optical element according to theinvention having rounded prism apexes. The brightness enhancing article330 features a flexible, base layer 332 having a pair of opposedsurfaces 334, 336, both of which are integrally formed with base layer332. Surface 334 features a series of protruding light-diffusingelements 338. These elements may be in the form of “bumps” in thesurface made of the same material as layer 332. Surface 336 features anarray of linear prisms having blunted or rounded peaks 340 integrallyformed with base layer 332. These peaks are characterized by a chordwidth 342, cross-sectional pitch width 344, radius of curvature 346, androot angle 348 in which the chord width is equal to about 20-40% of thecross-sectional pitch width and the radius of curvature is equal toabout 20-50% of the cross-sectional pitch width. The root angle rangesfrom about 70-110 degrees, or from about 85-95 degrees, with root anglesof about 90 degrees being preferred. The placement of the prisms withinthe array is selected to maximize the desired optical performance.

Rounded prism apex brightness enhancing articles usually suffer fromdecreased gain. However, the addition of high refractive index surfacemodified colloidal nanoparticles of the invention offsets the lost gainfrom the rounded prism apex brightness enhancing articles.

FIG. 9 shows another embodiment of an optical element according to theinvention having flat or planar prism apexes. The brightness enhancingarticle 430 features a flexible, base layer 432 having a pair of opposedsurfaces 434, 436, both of which are integrally formed with base layer432. Surface 434 features a series of protruding light-diffusingelements 438. These elements may be in the form of “flat bumps” in thesurface made of the same material as layer 432. Surface 436 features anarray of linear prisms having flattened or planar peaks 440 integrallyformed with base layer 432. These peaks are characterized by a flattenedwidth 442 and cross-sectional pitch width 444, in which the flattenedwidth can be equal to about 0-30% of the cross-sectional pitch width.

Another method of extracting light from a lightguide is by use offrustrated total internal reflection (TIR). In one type of frustratedTIR the lightguide has a wedge shape, and light rays incident on a thickedge of the lightguide are totally internally reflected until achievingcritical angle relative to the top and bottom surfaces of thelightguide. These sub-critical angle light rays are then extracted, ormore succinctly refract from the lightguide, at a glancing angle to theoutput surface. To be useful for illuminating a display device, theselight rays must then be turned substantially parallel to a viewing, oroutput, axis of the display device. This turning is usually accomplishedusing a turning lens or turning film.

FIGS. 10-12 illustrate an illumination device including a turning film.The turning film can include the inventive material disclosed herein forform a durable turning film. A turning lens or turning film typicallyincludes prism structures formed on an input surface, and the inputsurface is disposed adjacent the lightguide. The light rays exiting thelightguide at the glancing angle, usually less than 30 degrees to theoutput surface, encounter the prism structures. The light rays arerefracted by a first surface of the prism structures and are reflectedby a second surface of the prism structures such that they are directedby the turning lens or film in the desired direction, e.g.,substantially parallel to a viewing axis of the display.

Referring to FIG. 10, an illumination system 510 includes opticallycoupled a light source 512; a light source reflector 514; a lightguide516 with an output surface 518, a back surface 520, an input surface 521and an end surface 522; a reflector 524 adjacent the back surface 520; afirst light redirecting element 526 with an input surface 528 and anoutput surface 530; a second light redirecting element 532; and areflective polarizer 534. The lightguide 516 may be a wedge or amodification thereof. As is well known, the purpose of the lightguide isto provide for the uniform distribution of light from the light source512 over an area much larger than the light source 512, and moreparticularly, substantially over an entire area formed by output surface518. The lightguide 516 further preferably accomplishes these tasks in acompact, thin package.

The light source 512 may be a CCFL that is edge coupled to the inputsurface 521 of the lightguide 516, and the lamp reflector 514 may be areflective film that wraps around the light source 512 forming a lampcavity. The reflector 524 backs the lightguide 516 and may be anefficient back reflector, e.g., a lambertian or a specular film or acombination.

The edge-coupled light propagates from the input surface 521 toward theend surface 522, confined by TIR. The light is extracted from thelightguide 516 by frustration of the TIR. A ray confined within thelightguide 516 increases its angle of incidence relative to the plane ofthe top and bottom walls, due to the wedge angle, with each TIR bounce.Thus, the light eventually refracts out of each of the output surface518 and the back surface 520 because it is no longer contained by TIR.The light refracting out of the back surface 520 is either specularly ordiffusely reflected by the reflector 524 back toward and largely throughthe lightguide 516. The first light redirecting element 526 is arrangedto redirect the light rays exiting the output surface 518 along adirection substantially parallel to a preferred viewing direction. Thepreferred viewing direction may be normal to the output surface 518, butwill more typically be at some angle to the output surface 518.

As shown in FIG. 11, the first light redirecting element 526 is a lighttransmissive optical film where the output surface 530 is substantiallyplanar and the input surface 528 is formed with an array 536 of prisms538, 540 and 542. The second light redirecting element 532 may also be alight transmissive film, for example a brightness enhancing film such asthe 3M Brightness Enhancement Film product (sold as BEFIII) availablefrom Minnesota Mining and Manufacturing Company, St. Paul, Minn. Thereflective polarizer 534 may be an inorganic, polymeric, cholestericliquid crystal reflective polarizer or film. A suitable film is the 3MDiffuse Reflective Polarizer film product (sold as DRPF) or the SpecularReflective Polarizer film product (sold as DBEF), both of which areavailable from Minnesota Mining and Manufacturing Company.

Within array 536, each prism 538, 540 and 542 may be formed withdiffering side angles as compared to its respective neighbor prisms.That is, prism 540 may be formed with different side angles (angles Cand D) than prism 538 (angles A and B), and prism 542 (angles E and F).As shown, prisms 538 have a prism angle, i.e., the included angle, equalto the sum of the angles A and B. Similarly, prisms 540 have a prismangle equal to the sum of the angles C and D, while prisms 542 have aprism angle equal to the sum of the angles E and F. While array 536 isshown to include three different prism structures based upon differentprism angle, it should be appreciated that virtually any number ofdifferent prisms may be used.

Prisms 538, 540 and 542 may also be formed with a common prism angle butwith a varied prism orientation. A prism axis “l” is illustrated in FIG.11 for prism 538. The prism axis l may be arranged normal to the outputsurface 530, as shown for prism 538, or at an angle to the outputsurface either toward or away from the light source as illustrated byphantom axes “l⁺” and “l⁻”, respectively, for prisms 540 and 542.

Prisms 538, 540 and 542 may be arranged within array 536 as shown inFIG. 11 in a regular repeating pattern or clusters 543 of prisms, andwhile the array 536 is not shown to have like prisms adjacent likeprisms, such a configuration may also be used. Moreover, within thearray 536, the prisms 538, 540 and 542 may change continuously from afirst prism configuration, such as prism configuration 538, to a secondprism configuration, such as prism configuration 540, and so on. Forexample, the prism configuration may change in a gradient manner fromthe first prism configuration to the second prism configuration.Alternatively, the prisms may change in a step-wise manner, similar tothe configuration shown in FIG. 11. Within each cluster 543, the prismshave a prism pitch, which is selected to be smaller than the spatialripple frequency. Likewise, the clusters may have a regular clusterpitch. The prism array can be symmetrical as shown in FIG. 11 or theprism array can be non-symmetrical.

While the array 536 shown in FIG. 11 has prisms having a symmetricconfiguration, an array of prisms, such as array 536′ shown in FIG. 12formed in light redirecting element 526′, may be used. Referring then toFIG. 12, in the array 536′, prisms 538′, for example, has angle A′unequal to angle B′. Similarly for prisms 540′ and 542′, angle C′ isunequal to angle A′ and angle D′, and angle E′ is unequal to either ofangle A′, angle C′ or angle F′. The array 536′ may be advantageouslyformed using a single diamond cutting tool of a predetermined angle, andtilting the tool for each cut producing prisms of differing prism angleand symmetry. However, it will be appreciated that with the use of asingle cutting tool, the prism angles will be the same, i.e.,A+B=C+D=E+F.

It is contemplated that as few as two different prism configurations maybe used and arranged in clusters within the array 536, although as manyprism sizes as necessary to accomplish a modification of the outputprofile from the lightguide 516 may be used. One purpose of the prismside angle variation is to spread and add variable amounts of opticalpower into the first light redirecting element 526. The varyingconfiguration of prisms 538, 540 and 542 serves to provide substantiallyuniform sampling of the input aperture of the lightguide, whichminimizes non-uniformities in the light extracted from the lightguide516. The net result is an effective minimization of the ripple effectparticularly near the entrance end 521 of the lightguide 516.

The present invention should not be considered limited to the particularexamples described herein, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention can be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the instant specification.

EXAMPLES Preparation of the Optical Element

Compositions used in the preparation of exemplary optical elements ofthe present invention are described in the examples set forth below.Pertinent physical properties of the optical elements are described inthe tables set forth below. The preparation of exemplary opticalelements containing prismatic microstructures was similar to thosedescribed in U.S. Pat. Nos. 5,175,030, 5,183,597 and 7,041,365.

Unless otherwise specified, micro-prismatic structures have a 90° apexangle as defined by the slope of the sides of the prisms with the meandistance between adjacent apices being about 50 micrometers. The prismvertices are described as “sharp” (i.e., angular apex), “round” (apexhas a radius), or “flat” (i.e., prism is truncated).

All proportions shown in the examples are percent by weight unlessotherwise specified.

Scratch Test Method

Scratch testing on the samples formed, described below, was performedusing the following method. An optical film sample, an exemplary opticalelement of the present invention, 14 cm×17.8 cm (5.5 in×7 in), wasaffixed to a flat glass plate using adhesive tape on the 14 cm sides.For samples with microstructured grooves, the grooves were alignedparallel to the 17.8 cm side of the film sample. Using a stylus-typescratch test machine, the force on the stylus was adjusted to create ascratch on the surface of the sample approximately 30 to 200 micrometerswide. Choice of test variables such as stylus tip geometry or hardness,force applied to the stylus and stylus travel speed will depend onproperties of the sample to be tested and desired range of resultantvalues. Suitable scratch test machines include Taber Linear AbraderTester Model 5700 (Taber Industries, North Tonawanda, N.Y.), K9300Scratch Tester (Koehler Industries Co., Bohemia, N.Y.), and MicroScratch Tester (Micro Photonics, Inc., Irvine, Calif.). In general,pointed styli create more reproducible and readable scratches thanround-tipped styli.

For the samples described below, force applied to the stylus wasgenerally in the range of 8,000-40,000 g/cm². Scratches of sufficientlength to permit six separate width measurements were created in twoseparate areas of the sample. Scratch direction was orthogonal to theparallel ridges on the microstructured film sample. Scratch width wasmeasured by means of an optical microscope such as an Olympus MicroscopeModel BX51 at 20× magnification. For each scratch, six individualmeasurements of the scratch width were made along the length of thescratch. The six width measurements thus obtained were averaged for eachscratch line created. The average measurements taken from the twoseparate areas were then averaged to give an overall scratch widthaverage for each sample.

The lower limit of measurement of a scratch width is about 30micrometers by the above method. Results of this method for each exampleformed below are reported in the RESULTS section below. Because ofvariability inherent in the scratch test method, results may vary fromtest to test. It is recommended that a known control material is used asa standard. Performance of the sample material may then be compared tothat of the control.

Gain Test Method

Gain, the difference in transmitted light intensity of an opticalmaterial compared to a standard material, was measured on a SpectraScan™PR-650 SpectraColorimeter available from Photo Research, Inc,Chatsworth, Calif. Results of this method for each example formed beloware reported in the RESULTS section below. Film samples are cut andplaced on a Teflon light cube which is illuminated via a light-pipeusing a Foster DCR II light source.

Measurement of Refractive Index

Unless otherwise specified, all measurements of refractive index wereconducted on an Abbe Refractometer generally in accord with themanufacturer's recommendations and good laboratory practices. Results ofthis method for each example formed below are reported in the RESULTSsection below.

Determination of Metal Oxide Content

Metal oxide content, i.e., particle loading, of modified oxidedispersions and oxide-containing resin dispersions was determined bythermal gravimetric analysis (TGA). Results of this method for eachexample formed below are reported in the RESULTS section below.

LIST OF MATERIALS USED IN THE EXAMPLES BELOW

MATERIAL SOURCE DESCRIPTION 1-Methoxy-2-propanol Commodity Solvent2-[2-(2-methoxyethoxy) Sigma-Aldrich Aldrich catalog #40, 700-3ethoxy]acetic acid Milwaukee, WI (MEEAA) (Silane A174) Sigma-AldrichAldrich catalog #44015-9 3-(trimethoxysilylpropyl) Milwaukee, WI Silanesurface modifier methacrylate CN 120 Sartomer Co. Bisphenol-A epoxyExton, PA diacrylate oligomer Darocure 1173 Ciba SpecialtyPhotoinitiator Chemical, Inc. Tarrytown, NY Methacrylic acidSigma-Aldrich Milwaukee, WI Nalco 2327 Ondeo-Nalco Co. Colloidal silicadispersion Naperville, IL Nalco OOSSOO8 Ondeo-Nalco Co. Colloidalzirconia dispersion Naperville, IL SR 295 Sartomer Co. Pentaerythritoltetraacrylate Exton, PA monomer SR 339 Sartomer Co. 2-Phenoxyethylacrylate Exton, PA monomer Prostab 5128 Ciba Specialty Hindered aminenitroxide Chemical, Inc. inhibitor Tarrytown, NY Silquest A1230 OSISpecialties- Silane surface modifier Crompton South Charleston, WV TPOBASF Corp. Photoinitiator Mount Olive, NJ Optical Resin C 48 partsSartomer SR 295 (by weight) 35 parts Sartomer CN 120 (by weight) 17parts Sartomer SR 339 (by weight)

Preparation of Silane-Modified PEG

“PEG2TES” refers to N-(3-triethoxysilylpropyl)methoxyethoxyethylcarbamate. It was prepared as follows: A 250 ml round-bottomed flaskequipped with a magnetic stir bar was charged with 35 g diethyleneglycol methyl ether and 77 g methyl ethyl ketone followed by rotaryevaporation of a substantial portion of the solvent mix to remove water.3-(Triethoxysilyl)propylisocyanate (68.60 g) was charged to the flask.Dibutyltin dilaurate (approx. 3 mg) was added and the mixture stirred.The reaction proceeded with a mild exotherm. The reaction was run forapproximately 16 hr at which time infrared spectroscopy showed noisocyanate. The remainder of the solvent and alcohol were removed viarotary evaporation at 90° C. to yield 104.46 g PEG2TES as a somewhatviscous fluid.

Example 1A Silane-Modified Silica Nanoparticle Dispersion

Preparation of silane-modified silica nanoparticle dispersion: Nalco2327 (400.01 g) was charged to a 1 liter (32 oz) jar.1-Methoxy-2-propanol (450.05 g), Silane A174 (19.02 g), PEG2TES (9.45g), and 2% Prostab 5128 in H₂O (0.5 g) were mixed together and added tothe colloidal dispersion while stirring. The jar was sealed and heatedto 80° C. for 16.5 hr resulting in a clear, low viscosity dispersion ofsurface modified colloidal silica nanoparticles.

Example 1B Silane-Modified Silica Resin Dispersion

Preparation of silane-modified silica nanoparticle resin dispersion:Into a 1 liter round-bottom flask, 876.4 g of this modified silicadispersion (from Example 1A) was charged followed by 245.83 g OpticalResin C, and 4.9 g of a 2% solution of Prostab 5128 in water. Water andalcohol were removed by rotary evaporation at 80° C. thereby yielding aclear, low viscosity liquid resin composition containing approximately38.5% SiO2.

A total of ten batches were made and combined to yield 3,864.9 g ofsilane-modified colloidal silica nanoparticle resin. To this was added38.6 g Darocure 1173. The resultant surface-modified colloidal silicananoparticle containing resin contained 37.33% SiO2. The refractiveindex was 1.50.

Example 2 Silane-Modified Silica Resin Dispersion

Nalco 2327(1200.00 g) was charged to a 2 liter Ehrlenmeyer flask.1-Methoxy-2-propanol (1350.3 g), Silane A174 (57.09 g), and PEG2TES(28.19 g) were mixed together and added to the colloidal dispersionwhile stirring. The contents of the flask were poured into 3 32 ozsealed jars. The jars were heated at 80° C. for 16 hours. This resultedin a clear, low viscosity dispersion of surface modified colloidalsilica nanoparticles.

A 10 liter round-bottom flask (large neck) was charged with the contentsof the three jars (2638 g), 743.00 g Optical Resin C, and 8.0 g Prostab5128 at 2% in water. Water and alcohol were removed via rotaryevaporation. A clear, low viscosity resin dispersion containing surfacemodified colloidal silica nanoparticles was thus obtained. The resindispersion contained approximately 38.5% SiO2 and approximately 2%1-methoxy-2-propanol as measured by gas chromatography.

Example 2A

One percent by weight of Darocure 1173 was added to the resin dispersionof Example 2

Example 2B

The surface-modified colloidal nanoparticle dispersion of Example 2 wasdiluted with Optical Resin C to obtain a 28% dispersion. Darocure 1173was then added at 1% by weight.

Example 2C

The surface-modified colloidal nanoparticle dispersion of Example 2 wasdiluted with Optical Resin C to obtain an 18% dispersion. Darocure 1173was then added at 1% by weight.

Example 3 Silane-Modified MEEAA-Modified Zirconia Resin Dispersion

A flask was charged with 400.11 grams Nalco OOSSOO8 zirconia sol and32.84 grams MEEAA thus providing a surface coverage of 1.0 mmol MEEAAper gram of zirconia. This mixture was rotary evaporated to dryness anddried further in a vacuum oven at 90° C. to remove excess acetic acid.The powder thus obtained was redispersed in deionized water to give 30weight percent MEEAA surface-modified colloidal zirconia nanoparticles.A jar was charged with 150 grams of the zirconia nanoparticledispersion, and 11.41 grams Silane A174 dispersed in 150 grams of1-methoxy-2-propanol was added slowly with stirring thus providingsurface coverage at 1.0 mmol silane per gram of zirconia. The mixturewas heated at 90° C. for 2 hours.

A flask was charged with 152.43 grams of the silane-modified zirconiananoparticle dispersion, 30.54 grams Optical Resin C, and 0.47 gramProstab 5128 at 10% in water. The mixture was rotary evaporated at 80°C. until no solvent was being removed (approximately 30 minutes). Thesurface-modified colloidal zirconia nanoparticle resin dispersion thusobtained had a refractive index of 1.572.

Example 4 Silane Modified Zirconia Dispersion

Preparation of silane-modified zirconia nanoparticle dispersion: NalcoOOSSOO8 zirconia sol (456.02 g) and 41.18 g MEEAA were charged to a 1liter round bottom flask. The water and acetic acid were removed viarotary evaporation at 80° C. The powder thus obtained was redispersed in285 g D.I water and charged to a 2 liter beaker to which was added withstirring 625 g 1-methoxy-2-propanol, 63.11 g Silane A-174 and 42.31 gSilquest A-1230. This mixture was stirred 30 min at room temperaturethen poured into 1 liter (quart) jars, sealed and heated to 90° C. for4.5 h. The contents of the jars were removed and concentrated via rotaryevaporation to 570 g. Deionized water (1850 g) and 61.3 g concentratedaqueous ammonia (29% NH3) were charged to a 4 liter beaker. Theconcentrated dispersion was added slowly to the beaker with stirring.The white precipitate thus obtained was isolated via vacuum filtrationand washed with additional D.I. water. The damp solids were dispersed in540 g 1-methoxy-2-propanol. The resultant silane modified zirconiadispersion contained 15.58% ZrO₂.

Preparation of silane-modified zirconia nanoparticle resin dispersion: A1 liter round-bottom flask was charged with 429.6 g of this modifiedzirconia dispersion, 81.8 g Optical Resin C, and 1.6 g Prostab 5128 at 2wt % in water. Water and alcohol were removed via rotary evaporation. Aslightly hazy low viscosity liquid was obtained. The resultantsilane-modified zirconia resin dispersion contained 38.52% ZrO2 and hada refractive index of 1.57. Darocure 1173 (0.93 g) was added to theformulation.

Example 5 Mixed Modified Silica/Modified Zirconia Resin

A 1 liter round-bottom flask was charged with 250 g of the modifiedzirconia dispersion of Example 6, 106.72 g of the modified silicadispersion of Example 2, 50.00 g Optical Resin C, and 1.0 g Prostab 5128at 2% in water. Water and alcohol were removed via rotary evaporation. Aslightly hazy low viscosity liquid was obtained. The resin dispersionthus obtained contained approximately 31.4% ZrO₂, 16.1% SiO₂ and 46.28%total inorganic material. The dispersion had a refractive index of 1.55.Darocure 1173 (0.54 g) was added to the formulation.

Example 6 Silane-Modified Zirconia/Titania Resin Dispersion

A flask was charged with 200.43 grams of Optolake 3 zirconia coatedtitania sol (Catalysts & Chemical Industries Corp., Kawasaki, Japan) and6.05 grams of A1230 to give a surface coverage of 0.6 mmol/gram. Thiswas heated overnight at 90° C. A mixture of 200.24 grams of1-methoxy-2-propanol and Silane A174 were added slowly with mixing tothe treated zirconia/titania dispersion to give a surface coverage of0.6 mmol/gram. The resultant dispersion was heated overnight at 80° C.This was reduced on a rotary evaporator by 266.13 grams and then dilutedwith 113.46 grams 1-methoxy-2-propanol. This was reduced a second timeon a rotary evaporator by 121.54 grams and then diluted with 121.23grams 1-methoxy-2-propanol. This was finally reduced a third time by94.04 grams on a rotary evaporator. The resulting silane-treatedzirconia/titania dispersion was filtered through a 1 micron glass fibersyringe filter.

A flask was charged with 144.58 grams this mixture and combined with35.96 grams of Optical Resin C and 0.20 gram Prostab 5128 at 10% inwater. The mixture was rotary evaporated at 80° C. until no solvent wasbeing removed (approximately 30 minutes) thus providing a silane-treatedzirconia/titania resin dispersion with a refractive index of 1.576.

Example 7 MEEAA-Modified Zirconia Resin Dispersion

A flask was charged with 702.16 grams of Nalco OOSSOO8 zirconia sol and28.68 grams of MEEAA to give a surface coverage of 1.0 mmol/gram. Thiswas rotary evaporated to dryness and dried further in a vacuum oven at90° C. to remove excess acetic acid. The powder was redispersed indeionized water to provide a 23 weight percent dispersion ofMEEAA-treated zirconia. This was again rotary evaporated to dryness anddried further in a vacuum oven at 85° C. to remove excess acetic acid.The powder was redispersed in deionized water to give 23 weight percentzirconia. This solution was sonicated with a sonic horn for 44 minutesand then filtered through a 1 micron glass fiber sysringe filter.

A flask was charged with 326.19 grams of the MEEAA-treated zirconiadispersion, 321.52 grams of 1-methoxy-2-propanol, 90.11 grams of OpticalResin C and 0.34 gram Prostab 5128 at 10% in water. The mixture wasrotary evaporated at 80° C. until no solvent was being removed(approximately 30 minutes). This gave a resin with a refractive index of1.569.

Example 8 MEEAA-Modified Zirconia Resin Dispersion

A flask was charged with 702.16 grams of Nalco OOSSOO8 zirconia sol and28.68 grams of MEEAA to give a surface coverage of 1.0 mmol/gram. Themixture was rotary evaporated to dryness and dried further in a vacuumoven at 90° C. to remove excess acetic acid. The powder thus obtainedwas redispersed in deionized water to provide a 23 weight percentnanocolloidal dispersion of MEEAA-treated zirconia. This was againrotary evaporated to dryness and dried further in a vacuum oven at 85°C. to remove excess acetic acid. The powder was redispersed in deionizedwater to give 23 weight percent zirconia. This solution was sonicatedwith a sonic horn for 44 minutes and then filtered through a 1 micronglass fiber syringe filter.

A flask was charged with 349.91 grams of the MEEAA-treated zirconiadispersion, 4.63 grams of acrylic acid (yielding a surface coverage of1.0 mmol/gram), 340.55 grams of 1-methoxy-2-propanol, 97.12 grams ofOptical Resin C and 0.41 gram Prostab 5128 at 10% in water. The mixturewas rotary evaporated at 80° C. until no solvent was being removed(approximately 30 minutes). This gave a MEEAA surface modified zirconiananoparticle resin dispersion with a refractive index of 1.548.

Example 9 Silane-Modified Zirconia Resin Dispersion

A flask was charged with 352.67 grams of Nalco OOSSOO8 zirconia sol and23.99 grams of A1230 to give a surface coverage of 0.6 mmol/gram. Themixture was heated for 1 hour at 90° C. To this was added slowly using aseperatory funnel, a mixture of 36.57 grams of A174 dispersed in 350.72grams of 1-methoxy-2-propanol to give 1.8 mmol/gram surface coverage.The mixture was heated at 90° C. for 4 hours. The solution was reducedby 422.66 grams on a rotary evaporator. The solution was combined withan ammonia water solution to flocculate the zirconia in a final pH>9solution (700 grams deionized water mixed with 80 grams of 30 percentammonium hydroxide. The slurry was filtered on a 185 mm Whatman #4filter paper and the precipitate was washed with 350 grams deionizedwater. The precipitate was dispersed into 1-methoxy-2-propanol to give23 weight percent zirconia. This solution was sonicated with a sonichorn for 44 minutes and then filtered through a 1 micron glass fibersyringe filter.

A flask was charged with 331.80 grams this sol and combined with 46.51grams of Optical Resin C and 0.46 gram Prostab 5128 at 10% in water. Themixture was rotary evaporated at 80° C. until no solvent was beingremoved (approximately 30 minutes). This gave a resin with a refractiveindex of 1.541.

Example 10 Phenyl/Isooctyl/Methacrylate Silane Modified SilicaDispersion

Nalco 2327(199.9 g) was charged to a 16 oz jar. 1-Methoxy-2-propanol(400.02 g), 3-(trimethoxysilylpropyl)methacrylate (0.28 g), isooctyltrimethoxy-silane (0.59 g) and phenyltrimethoxysilane (9.44 g) weremixed together and added to the colloidal dispersion while stirring. Thejar was sealed and heated to 90 C for 22 hr. This resulted in a hazy,high viscosity solution of modified silica.

A 1 liter round-bottom flask (large neck) was charged with the abovemodified sol (405.4 g), OPTICAL RESIN C (82 g), and 2 wt % hinderedamine nitroxide in water (1.64 g). Water and alcohol were removed viarotary evaporation. A clear high viscosity liquid was obtained. Thesolution contained approximately 38.5 wt % SiO2. Darocure 1173 (1.3 g)was added.

COMPARATIVE EXAMPLES

The following comparative examples were formed using similar resinsystems and micro-structured topography and processes for forming thesame as the examples above. However, the comparative examples do notinclude surface modified colloidal nanoparticles.

Comparative Example A

Vikuiti™ BEF II 90/50 film (BEF II), sold by 3M, St Paul, Minn., is amicroreplicated prismatic structured brightness enhancing film having aprism angle of 90 degree and a pitch (distance between prism peaks) of50 micrometers. The prism peaks in Comparative Example A are sharp.

Comparative Example B

Vikuiti™ Rounded Brightness Enhancement Film (RBEF) film, sold by 3M, StPaul, Minn., is a microreplicated prismatic structured brightnessenhancing film having a prism angle of 90 degree and a pitch of 50micrometers. The prism peaks in Comparative Example B are rounded andhave a peak radius of 8 microns.

Comparative Example C

Darocure 1173 was added to Optical Resin C at 1% by weight and theresultant composition was cured against a microstructured tool surfaceas described in the references. Prism peaks of Comparative Example C canbe sharp, rounded or flat depending on the structure of the tool usedduring the cure of the resin.

Results

TABLE 1 COMPOSITION SUMMARY Particle Uncured Metal Loading Refr. ExampleOxide Modifier (%) Index COMP EX A None None  0 COMP EX B None None  0COMP EX C None None  0   1B SiO₂ A174 37 1.50 PEG2TES   2A SiO₂ A174 381.497 PEG2TES   2B SiO₂ A174 28 1.503 PEG2TES   2C SiO₂ A174 18 1.507PEG2TES   3 ZrO₂ MEEAA 39 1.572 A174   4 ZrO₂ A174 38 1.57 A1230    5SiO₂ See example 46 1.55 ZrO₂   6 ZrO₂/TiO₂ A1230  32 1.576 A174   7ZrO₂ MEEAA 35 1.569   8 ZrO₂ MEEAA 33 1.548   9 ZrO₂ A1230   33* 1.541A174 10 SiO₂ Phenyl/isooctyl 38 A174 *Estimated value

TABLE 2 GAIN TABLE Peak Peak Radius Ave EXAMPLE Type (micron) Gain CompEx A Sharp 0 1.716 Comp Ex C Sharp 0 1.627  2A Sharp 0 1.594  2B Sharp 01.608  2C Sharp 0 1.620 4  Sharp 0 1.699 5  Sharp 0 1.632

Table 2 illustrates particle loading, particle type, and resin type andtheir respective relationships to gain. In general, gain is a functionof index of refraction within the range of materials tested. That is,higher index of refraction results in higher gain.

TABLE 3 GAIN TABLE Peak Peak Radius Ave EXAMPLE Type (micron) Gain CompEx C Round 2 1.632  2A Round 2 1.590  2B Round 2 1.604  2C Round 2 1.6114  Round 2 1.665 5  Round 2 1.603 Comp Ex C Round 4 1.625  2A Round 41.579  2B Round 4 1.591  2C Round 4 1.603 4  Round 3 1.640 5  Round 31.584 Comp Ex C Round 10.5 1.504  2A Round 10.5 1.474  2B Round 10.51.484  2C Round 10.5 1.490 4  Round 6.6 1.587 5  Round 6.6 1.550

TABLE 4 GAIN TABLE Peak Peak Width Ave EXAMPLE Type (micron) Gain CompEx C Flat 2 1.623 2A Flat 2 1.577 2B Flat 2 1.587 2C Flat 2 1.600 CompEx C Flat 4 1.593 2A Flat 4 1.550 2B Flat 4 1.563 2C Flat 4 1.572

Tables 3 and 4 illustrates particle loading, particle type, resin type,peak type, peak radius or peak width and their respective relationshipsto gain. In general, gain is a function of index of refraction withinthe range of materials tested. That is, higher index of refractionresults in higher gain. Additionally, sharp-tip prisms result in maximumgain for a given composition, and usually, smaller peak radii result inhigher gain when compared to larger peak radii for round prism apexes.Also, flat or truncated prismatic apexes result in decreased gain whencompared to similar round geometries of the same composition.

TABLE 5A SCRATCH TESTING Ave Peak Scratch Radius Width Example (micron)(micron) COMP EX A 0 148 COMP EX C 0 129 2A 0 121 2B 0 124 2C 0 120

Table 5a illustrates the difference in scratch resistance for differentcompositions. In general, larger scratch width implies lower scratchresistance. Addition of inorganic oxide particles to the resin systemincreases the scratch resistance of the cured composition. Also, COMP EXC shows that the OPTICAL RESIN C is more scratch resistant than theresin of example COMP EX A.

TABLE 5B SCRATCH TESTING Ave Peak Scratch Radius Width Example (micron)(micron) SHARP COMP EX A 0 94  1B 0 78 4  0 88 5  0 80 6  0 90 7  0 888  0 90 9  0 88

Table 5b illustrates the difference in scratch resistance for differentcompositions. In general, larger scratch width implies lower scratchresistance. Addition of inorganic oxide particles to the resin systemincreases the scratch resistance of the cured composition. In general, ahigher volume loading of inorganic oxide particles decreases the averagescratch width.

TABLE 6 SCRATCH TESTING Ave Peak Scratch Radius Width Example (micron)(micron) ROUND 2B 2 112 2B 10.5 <30 2C 2 107 2C 10.5 <30 COMP 8.0 132 EXB COMP 2 128 EX C COMP 10.5 <30 EX C

Table 6 illustrates the effect of degree of prism apex rounding onscratch width. So, in general, smaller peak radii prism apexes result inmaximum scratch width for a given composition when compared to largerdegree peak of the same composition.

TABLE 7 SCRATCH TESTING Ave Peak Scratch Radius Width Example (micron)(micron) ROUND  1B 6.6 42 3  6.6 95 4  6.6 76 5  6.6 49 6  6.6 69 7  6.679 8  6.6 85 9  6.6 83

Table 7 illustrates the difference in scratch resistance for differentcompositions. Typically, larger scratch width implies lower scratchresistance. Addition of inorganic oxide particles to the resin systemincreases the scratch resistance of the cured composition. In general, ahigher volume loading of inorganic oxide particles decreases the averagescratch width.

TABLE 8 SCRATCH TESTING FLAT WIDTH Scratch width Example (micron)(micron) FLAT 1B 0 130 1B 2.5 114 1B 4 87

Table 8 illustrates the effect of degree of prism apex flat width ortruncation on scratch width. So, in general, smaller flat width prismapexes result in maximum scratch width for a given composition whencompared to larger flat width of the same composition.

TABLE 9 SCRATCH TESTING Ave Peak Scratch Radius Width Example (micron)(micron) ROUND   1B 10 119 10 10 144

Table 9 illustrates, in general, higher amounts of a surface modifier onthe particle containing a reactive end group that is copolymerizablewith the resin system results in smaller scratch widths. Typically,smaller scratch width implies greater scratch resistance.

Scratch width data have been presented as an indicator of durability ofexemplary optical elements. In general, the greater the width of ascratch pattern, the less durable the element. Without desiring to bebound by a particular theory, the inventors believe that the stylus ofthe scratch tester engages and penetrates the microstructured grooves atthe surface of a less durable optical element more easily therebyresulting in a relatively uniform scratch pattern. It is envisioned thata very low durability material would have a relatively large scratchwidth and a low variability statistic such as the standard deviation ofthe measurement.

It has also been observed that variability within the data, especiallyfor optical elements with microstructured surfaces, can also be anindicator of durability. Thus, the greater the variability of themeasurement, the more durable the optical element. It is thought that amore durable material is less easily engaged and penetrated by thestylus. As a result, the stylus does not track smoothly across themicrostructured elements but, in fact, is caused to “bounce” from oneset of grooves to another penetrating the surface of some morecompletely than others. The resultant scratch pattern has a combinationof shorter and longer scratches. Under this circumstance, it isenvisioned that a more durable optical element would present arelatively low scratch width with a greater variability statistic.

1. A durable optical film comprising: a polymerized optical filmstructure having a microstructured surface comprised of a polymerizedcomposition comprising a resin having a refractive index of at least1.50 and a plurality of surface modified colloidal nanoparticles ofzirconia wherein the zirconia has a primary particle size of 5 to 15 nm.2. The durable optical film according to claim 1 wherein the polymerizedoptical film structure has a plurality of ridges extending along a firstsurface.
 3. The durable optical film of claim 1 wherein the totalcomposition has a refractive index greater than 1.56.
 4. The durableoptical film according to claim 1 wherein the zirconia comprises 10 to70 wt % of the microstructured surface.
 5. The durable optical filmaccording to claim 1 wherein the zirconia comprises 30 to 50 wt % of themicrostructured surface.
 6. The durable optical film of claim 1 whereinthe plurality of surface modified colloidal nanoparticles furthercomprises silica nanoparticles.
 7. The durable optical film according toclaim 6 wherein the silica particle size is from 5 to 75 nm.
 8. Thedurable optical film according to claim 6 wherein the silica particlesize is from 10 to 30 nm.
 9. The durable optical film according to claim6 wherein the silica comprises 10 to 60 wt % of the microstructuredsurface.
 10. The durable optical film according to claim 6 wherein thesilica comprises 10 to 40 wt % of the microstructured surface.
 11. Thedurable optical film according to claim 1 wherein the surface modifiednanoparticles further comprise titania, antimony oxides, alumina, tinoxides, mixed metal oxides thereof, or mixtures thereof.
 12. The durableoptical film of claim 11 wherein the surface modified nanoparticlescomprises a mixed metal oxide of zirconia and titania.
 13. The durableoptical film according to claim 12 wherein the mixed metal oxideparticle size is from 5 to 15 nm.
 14. The durable optical film accordingto claim 12 wherein the mixed metal oxide comprises 10 to 70 wt % of themicrostructured surface.
 15. The durable optical film according to claim12 wherein the mixed metal oxide comprises 30 to 50 wt % of themicrostructured surface.
 16. The durable optical film of claim 1 whereinthe nanoparticles are substantially fully condensed.
 17. The durableoptical film according to claim 1 wherein the polymerized compositioncomprises a surface treatment agent selected from the group consistingof alcohols, amines, carboxylic acid, sulfonic acids, phosphonic, acids,silanes, titanates, and mixtures thereof.
 18. The durable optical filmaccording to claim 1 wherein the surface treatment agent comprises atleast one surface treatment agent that copolymerizes with the resin. 19.The optical film of claim 18 wherein the copolymerizable group is anethylenically unsaturated group.
 20. A device comprising: (a) a lightingdevice having a light-emitting surface; and (b) the brightness enhancingfilm of claim 1 placed substantially parallel to said light-emittingsurface.